It is generally known that antenna performance is dependent on the size, shape, and material composition of constituent antenna elements, as well as the relationship between the wavelength of the received/transmitted signal and certain antenna physical parameters (that is, length for a linear antenna and diameter for a loop antenna). These relationships and physical parameters determine several antenna performance characteristics, including: input impedance, gain, directivity, signal polarization, radiation resistance and radiation pattern.
Generally, an operable antenna should have a minimum physical antenna dimension on the order of a half wavelength (or a quarter wavelength above a ground plane) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted energy. Antennas having an operative dimension of a half wavelength or multiples thereof, are commonly used. Certain antennas present an electrical dimension that is not equivalent to a physical dimension of the antenna. Such antennas should therefore exhibit an electrical dimension that is a half wavelength (or a quarter wavelength above a ground plane) or a multiple thereof.
The burgeoning growth of wireless communications devices and systems has created a need for physically smaller, less obtrusive and more efficient antennas that are capable of wide bandwidth operation, multiple frequency band operation and/or operation in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns).
The relatively small packaging envelopes of current handheld communications devices may not provide sufficient space for the conventional quarter and half wavelength antennas. In many applications, the antenna is therefore mounted so as to protrude from the device package, subjecting the external protrusion to damage, especially when carried by the user or when stored within a carrying-case. Thus a physically smaller antenna for mounting within the compact environment of a handset package and operational in the frequency bands of interest is especially sought after. Such an antenna must also provide other desired antenna operating properties, e.g., matched input impedance, radiation pattern, signal polarizations, etc. To further complicate the antenna packaging issue, it is known to those skilled in the art that there is a direct relationship between antenna gain and antenna physical size. Increased gain requires a physically larger antenna, while handset manufacturers continue to demand physically smaller antennas with increased gain characteristics.
The electronic components of a wireless communications device are typically mounted on a printed circuit board enclosed within a case. To reduce the burden of incorporating the antenna into such devices, the antenna mounting structure should be compatible with printed circuit board fabrication and assembly techniques. Specifically, a surface-mount antenna configuration, i.e., wherein the antenna is mounted to a surface of the printed circuit board, permits relatively easy physical mounting and electrical connection of the antenna to the electronic components of the communications device.
U.S. Pat. No. 3,967,276 describes an antenna structure (the so called “Goubau” antenna) comprising four elongated conductors 1, 2, 3 and 4 (see FIG. 1) having dimensions and spacing that are small compared to a wavelength of the applied signal. The conductors are oriented perpendicular to a ground plane 13 with an upper end of each conductor terminated in a conductive plate, identified in FIG. 1 by reference characters 5, 6, 7 and 8. The plates 5, 6, 7 and 8 are spaced apart from each other (i.e., segmented) and capacitively coupled to the ground plane 13. The plates 6, 7 and 8 are oriented parallel to and electrically connected to the ground plane 13 via the conductors 2, 3 and 4. The plate 5 is connected to a signal source (in the transmitting mode) via the conductor 1. In the receiving mode a received signal is supplied to receiving circuitry (not shown), via the conductor 1. Adjacent ones of the plates 5, 6, 7 and 8 are connected by inductive elements 9, 10, 11 and 12.
The plates 5, 6, 7 and 8 and the inductive elements 9, 10, 11 and 12 can be dimensioned (with respect to the inductive elements, dimensioning refers to the inductance and resistance of each inductive element) and spaced apart such that the effective electrical length of the antenna is four times the physical height. For example, in one embodiment the antenna has a physical height of 2.67 inches. When operative with a signal having a wavelength of 60 cm (and thus a frequency of 500 MHz), the antenna is designed to present an effective electrical length of about 10.7 cm. The antenna also exhibits a radiation resistance of about 50 ohms for balancing to a standard transmission line having a 50 ohm impedance.
It is known that a monopole antenna comprising a single flat element spaced apart from a ground plane by a distance of about 2.67 cm has a radiation resistance of only about 3.1 ohms at the same operating frequency of 500 MHz. The antenna thus requires use of an impedance-matching transformer, which substantially reduces antenna efficiency and the radiation bandwidth. Use of the four plates 5, 6, 7 and 8 increases the radiation resistance by the factor N^2, where N is the number of plates.
Typically, the plates 5, 6, 7 and 8 are constructed from sheet metal material, with the elongated conductors 1, 2, 3 and 4 comprising conductive wire. These structures can be relatively expensive to fabricate and thus may not be suitable for use with handheld communications devices where manufacturing cost is an important factor. Also, forming and attaching the inductive elements 9, 10, 11 and 12 is a labor intensive process that is not easily implemented in a printed circuit board manufacturing process. Clearly, an antenna constructed according to the Goubau patent is not ideally suited for mounting on a printed circuit board of a communications handset device.